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Homework Help: Block falls on Spring! Work and Speed!

  1. Feb 1, 2005 #1
    A block of mass 240 grams is dropped onto a relaxed vertical spring that has a spring constant 11 N/cm as shown on the figure. The block becomes attached to the spring and compresses the spring 15 cm before momentarily stopping.

    First Conversions:
    240g = .24kg, 11 N/cm = 1100 N/m, 15cm = .15m

    While the spring is being compressed, what work is done on the block by the gravitational force on it?

    .24kg * 9.8 m/s^2 = 2.352 N
    2.352 N * .15m = .3528 J -> This is definitley correct

    What work is done on the block by the spring force?

    Work done over distance via a function so use integration:

    W = 0 to .15m ( 1100N/m x dx ) =
    W = 0 to .15m ( 1100N/m x^2/2 )
    W = (.15m)^2 / 2 * 1100N/m ) = 12.375 J -> No idea if this is right.

    What is the speed of the block just before it hits the spring? (Assume that friction is negligible.)

    This is where I have problems, I have NO idea how to find the speed of the block. I don't know how high it falls from, I tried using 9.8m/s^2 as an accel and VF = 0 in vf^2 = vi^2 + 2ad, but no go, can anyone shed light on this part?

    If the speed at impact is doubled, what is the maximum compression in the spring?

    No idea on this either, I don't know the answer to part 3 is probably why.

    Any help on this would be great!
  2. jcsd
  3. Feb 1, 2005 #2
    I managed to get part 3 and 4, but the Part 2 with 12.375 J is still wrong, It looks perfect to me, did I miss something?
  4. Feb 1, 2005 #3
    Got it all even though no one helps, thanks anyway lol, someone delete this please?
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